This invention concerns light source equipment that includes what is called a dielectricbarrier discharge lamp, which is a type of discharge lamp used, for example, as a source of ultraviolet radiation for photochemical reactions, in which excimer molecules are formed by dielectric-barrier discharge, and which uses light emitted from the excimer molecules.
Technical literature explaining the technology involved in the dielectric-barrier discharge lamps with which this invention is concerned can be found in, for example, JPO kokai patent report H2-7353. This document describes an emitter that produces light by causing the formation of excimer molecules by means of a dielectric barrier discharge (also known as ozonizer discharge or silent discharge; see Denki Gakkai, xe2x80x9cDischarge Handbook,xe2x80x9d revised edition, 7th printing, June 1989, p. 263) in a discharge chamber filled with a discharge gas that forms excimer molecules, and using the light radiated by those excimer molecules.
Dielectric-barrier discharge lamps as described above, and light source equipment containing such lights, have a number of advantages not found in conventional low-pressure mercury discharge lamps and high-pressure arc discharge lamps, and so have a variety of potential applications. One of the most important of these, given the mounting interest in the issue of environmental pollution in recent years, is the decontamination of materials by means of photochemical reactions using ultraviolet radiation. Consequently, there is unusually strong demand for dielectric-barrier discharge lamp light source equipment with high outputs and broad areas of illumination.
One proposal in line with this demand is found in JPO kokai patent H4-229671, which describes a constitution that enlarges the light source and expands the area of illumination by lighting multiple dielectric-barrier discharge lamp in parallel. There are, however, a number of major, unresolved problems in such attempts to use conventional technology. The first problem is that it is difficult, when illuminating a broad area, to make the illumination energy density uniform or to make the light adjustable. The second problem is greater economy is sought as the output is increased and the area of illumination is enlarged, or in other words as the electrical power of the equipment is increased. The third problem is that, as the output is increased and the area of illumination is enlarged, the heat generated by the lamp increases and its service life grows shorter.
Now, the reason for the necessity of being able to adjust the light within the first problem is simply explained. The function of treating materials using ultraviolet light from dielectric-barrier discharge lamps depends on highly complicated and precise photochemical reactions; in order to obtain the desired treatment effect in materials of large area, it is necessary that the illumination energy density distribution not be greater or less than the desired distribution. In the event that the illumination energy density is inadequate, the effect of illumination is low, which is clearly a problem. In the event that the illumination energy density is excessive, problems are caused by excessive reactions that go beyond the proper limits. For example, the breakdown products of the ultraviolet light illumination may react again and undergo unintended molecular synthesis, or an uneven layer of impurities may be formed on the surface of the material being treated. Accordingly, there is a permissible range that depends on the sort of treatment to be performed, and to avoid illumination energy density distribution that is not greater or less than the desired distribution, the ideal dielectric-barrier discharge lamp should have the function of adjusting the illumination energy density to fit the permissible range.
Moreover, in dielectric-barrier discharge lamps, as in other lamps, there are variable factors in the intensity of light emitted. The first variable factor is variation of the period of time needed for electrical and thermal stabilization after the lamp is lighted. The second variable factor is the period between the lamp being in new condition to the end of its service life. The function of adjusting the illumination energy density is needed to correct for these variables and maintain the desired illumination energy density.
A proposal to resolve the first problem mentioned above, which is the difficulty when illuminating a broad area or making the illumination energy density uniform and making the light adjustable, was made in JPO kokai patent H8-146198, but that did not include positive solutions to the second problem or the third problem.
In order to resolve the third problem mentioned above, it is necessary to improve the lighting efficiency of the lamp. The conditions for improvement of the lighting efficiency of lamps are explained below.
Dielectric-barrier discharge lamps (B, B1, B2 . . . ) have a discharge plasma space (G) and one or two dielectrics sandwiched between electrodes (Ea, Eb). FIG. 1 shows a single dielectric-barrier discharge lamp with two dielectrics (D). In FIG. 1, by the way, the lamp seal (6) also serves as the dielectric (D).
When lighting up the dielectric-barrier discharge lamp (B), a high-frequency, alternating current of, for example 10 to 200 kHz and 2 to 10 kV is impressed on the electrodes (Ea, Eb). However, because of the dielectric (D) between the discharge plasma space (G) and the electrodes (Ea, Eb), current does not flow directly from the electrodes (Ea, Eb) to the discharge plasma space (G); the current flows by means of the action of the dielectric (D) as a condenser. In other words, a charge equal in size and opposite in sign to that on electrodes (Ea, Eb) is induced on the discharge plasma space side of the dielectric (D) because of polarization of the dielectric; The discharge occurs between the dielectric (D) that faces across the discharge plasma space (G).
Little current flows along the discharge plasma space (G) side of the dielectric (D); when discharge occurs, the charge induced on the discharge plasma space (G) side of the dielectric (D) is neutralized by the charge moved by the discharge, and the electrical field within the discharge plasma space (G) is reduced. For that reason, the current stops even if the voltage continues to be impressed on the electrodes (Ea, Eb). But when the voltage impressed on the electrodes (Ea, Eb) rises again, the discharge current continues. When the discharge ceases after having occurred, there is no further discharge until the polarity of the voltage impressed on the electrodes (Ea, Eb) has reversed.
In the case of a dielectric-barrier discharge lamp in which xenon gas, for example, is sealed, the xenon gas is dissociated into ions and electrons by the discharge, and becomes xenon plasma. When the xenon plasma is excited to a specified energy level, excimer molecules are formed within the plasma. Xenon excimers divide after a certain lifespan, but the energy released at that time is emitted as a photon of vacuum ultraviolet wavelength. To make a dielectric-barrier discharge lamp work efficiently as a vacuum ultraviolet light source, it is necessary to form the excimer molecules efficiently.
The greatest obstacle to efficient formation of excimer molecules during discharge is the excitation of the discharge plasma to energy levels that do not contribute to the formation of excimer molecules.
The movement of discharge plasma electrons immediately after discharge begins is collective, and the energy is high but the temperature is low. In this state, the discharge plasma has a high probability of transition to the resonant state required for formation of excimer molecules. If the discharge time is prolonged, however, the movement of the plasma electrons gradually becomes thermal. That is, it reaches a state of thermal equilibrium known as a Maxwell-Boltzmann distribution; the plasma temperature rises, and there is an increased probability of transition to a state of higher excitation where excimer molecules cannot form.
Moreover, sometimes when excimer molecules have been formed, a subsequent discharge will break down the excimer molecules before their lifespan elapses and they divide naturally by emitting the desired photon. In fact, in the case of xenon excimers, a period of about 1 ps is required between the beginning of discharge and emission of a vacuum ultraviolet photon, and a subsequent discharge or redischarge during that period reduces the efficiency of excimer light emission.
In other words, once discharge had commenced, it is most important to reduce as much as possible the energy of subsequent discharges.
Even in the event that the discharge time is short, if the energy injected during the discharge period is too great, there is similarly an increased probability of transition to a state of high excitation. Plasma that has transitioned to a state of high excitation moderates itself by emission of infrared radiation, which just raises the temperature of the lamp and does not contribute to excimer light emission.
That is, the discharge must be driven so as to suppress the excitation of discharge plasma to energy levels that do not contribute to the formation of excimer molecules. That point is one that cannot be satisfied by conventional dielectric-barrier discharge lamp light source equipment.
JPO kokai patent report H1-243363 is a proposal to achieve excimer light emission with high efficiency by means of all pulse discharges, including dielectric-barrier discharges. This follows the condition stated above that once a discharge has begun, the energy of the subsequent discharge is reduced as much as possible. However, what is described in this proposal is which parameters to control to increase the efficiency of excimer light emission; there is no specific mention of the effective conditions for those parameter values. Particularly in the case of dielectric-barrier discharges, there is little freedom for control of the voltage that has to be impressed and the current that has to be injected into the discharge plasma space through the dielectric; it is extremely difficult to discover the optimum conditions, but no information on that is contained in this proposal.
Proposals to improve the drive waveform of fluorescent lights using dielectric-barrier discharge include JPO kokai patent H6-163006. That describes improvement of the luminance of fluorescent lights by driving them with a stream of short pulses of positive and negative polarity or with alternating-current short waveforms. In connection with the frequency and duty cycle, experimental results on variation of luminance relative to variation of the voltage impress are described, and it is explained that efficiency is improved relative to conventional sine-wave drive. Practical power supplies, however, contain high-voltage transformers, and it is impossible to impress an ideal stream of short pulses or short waveforms; because of interaction of the output impedance of the power supply and the inductance of the lamp, the waveform is rounded and resonance causes a partially sine-wave shaped voltage to be impressed. With the premise that there is inevitable divergence from the ideal short waveform in practical power supplies of this sort, it is not easy to design and manufacture an economical and useful light source while keeping the harmful component of the divergence within acceptable limits, and that proposal does not state specific guidelines for resolving that problem.
There are proposals to improve the efficiency of dielectric-barrier discharge lamps, such as JPO patent report H8-508363. However, this proposal says nothing about specific items that are truly effective in achieving control of the excitation of discharge plasma to energy levels that do not contribute to formation of excimer molecules, so as to form excimer molecules efficiently.
In order to form excimer molecules efficiently while limiting the excitation of discharge plasma to energy levels that do not contribute to the formation of excimer molecules, it is best to raise the voltage impressed on the lamp at a finite rate of increase and to end the discharge as quickly as possible once the voltage for commencement of discharge is reached and discharge begins.
The operation of the electrical circuitry of a dielectric-barrier discharge lamp is shown in FIG. 2. The discharge path (7) of the discharge plasma space (G) can be thought of as connected in series with a resistor (8) and a switch (9). The dielectric-barrier discharge lamp (B) has dielectric (D) between the electrodes (Ea, Eb) and the discharge plasma space (G), and it functions as a condenser within the electrical circuitry. In the event that there are two pieces of dielectric, however, the two condensers can be thought of as a single condenser (10).
Because the structure has this condenser inserted in series with the discharge plasma space (G), discharge current flows through the dielectric-barrier discharge lamp (B) only for a period immediately after the change in polarity of the voltage impressed on the lamp, and a non-discharge period occurs naturally, even without impressing voltage on the lamp as a pulse voltage that has a rest period that is essentially zero voltage. Moreover, discharge does not occur until the voltage of the discharge plasma space (G) reaches the voltage for the commencement of discharge, and so it is not necessary for the rise or fall of the voltage impressed on the lamp to be rapid.
The discharge plasma space (G) itself forms a condenser (11), and when discharge begins, almost all the energy stored in this condenser is expended, and so once the discharge begins, there is no need for additional current to the dielectric-barrier discharge lamp (B) from the power supply.
The unit area of the lamp wall surface is considered next. The voltage for the commencement of discharge is decided almost automatically when the gas pressure and the discharge gap are decided. And because the size of the discharge gap determines the static capacity of the condenser (11) formed by the discharge plasma space, the minimum energy that can be put into the plasma between the commencement and completion of a single discharge is the energy of a full discharge of the charge stored in the condenser (11) formed by the discharge plasma space. That determines the constitution of the lamp. It was stated above that in order to form excimer molecules efficiently, the excitation of discharge plasma at energy levels that will not contribute to the formation of excimer molecules is controlled. That control is best achieved through the conditions for discharge of that minimum energy. However, the conditions for discharge of that minimum energy can be realized in an ideal fashion by very slowly raising the voltage impressed on the lamp, and then discharging it, using a power supply with an output impedance that is quite large.
There are, however, a number of problems in applying such power supply equipment as actual light source equipment. That is, there is the problem that because of the cyclical repetition of the discharge it is difficult to achieve the speed of operation when the output impedance is large, and the problem that under the conditions for discharge of the minimum energy, there is liable to be a lack of uniformity of discharge within each individual lamp because of the lack of uniformity of the location of the discharge gap within the lamp.
This latter problem is none other than the first of the problems listed previously, that it is difficult, when illuminating a broad area, to make the illumination energy density uniform or to make the light adjustable. That is, if one attempts to improve the lighting efficiency of the lamp in order to resolve the third problem listed previously, which is that as the output is increased and the area of illumination is enlarged the heat generated by the lamp increases and its service life grows shorter, one will actually exacerbate the first problem, and consequently it is extremely difficult to resolve both problems simultaneously.
Accordingly, in order to have practical light source equipment that uses a means of power supply with an output impedance small enough to realize the necessary light volume and that has surplus capacity to produce an even discharge at all surfaces of the dielectric-barrier discharge lamp, it is necessary to increase the voltage impressed on the lamp beyond the conditions for the minimum energy discharge mentioned above, and also to set correctly the conditions for the permissible range for lowering the efficiency of excimer emission by raising the voltage impressed on the lamp. Normally the appropriate range for the voltage impressed on the lamp is not a broad one.
In dealing with the first problem mentioned above, it is necessary to consider not only the problem of the lack of uniformity of discharge within each individual lamp because of the lack of uniformity of the location of the discharge gap within the lamp, but also the problem of lack of uniformity caused by differences of lighting efficiency among the different lamps.
The only way to make up for this lack of uniformity caused by differences of lighting efficiency among the different lamps is to adjust individually the power injected into each lamp. As stated above, however, the appropriate range for the voltage impressed on the lamp is not broad, and so it is not appropriate to adjust the power injected into each lamp by means of the voltage impressed on the lamp. That is because if the voltage impressed on the lamps is increased and decreased to correct for the lack of uniformity caused by differences of lighting efficiency among the different lamps, the voltage will usually fall outside the appropriate range.
The task to be handled by this invention is to provide a dielectric-barrier discharge lamp light source equipment to solve the difficulty, when illuminating a broad area, of making the illumination energy density uniform or making the light adjustable, and to solve at the same time the problem that as the output is increased and the area of illumination is enlarged, the heat generated by the lamp increases and its service life grows shorter, and that will achieve these solutions economically.
In order to resolve these problems, the dielectric-barrier discharge lamp light source equipment of this invention has the following constitution.
(1) Dielectric-barrier discharge lamp light source equipment comprising dielectric-barrier discharge lamps (B, B1, B2 . . . ) that have a discharge plasma space (G) filled with a discharge gas in which a dielectric-barrier discharge will produce excimer molecules and a dielectric (D) located between the discharge gas and at least one of two electrodes (Ea, Eb) to induce the discharge phenomenon in the discharge gas, and also a power supply to impress a high voltage on the electrodes (Ea, Eb) of the dielectric-barrier discharge lamps, in which the power supply means is divided into a power supply front stage (M) and power supply back stages (S, S1, S2 . . . ) with one of the power supply back stages (S, S1, S2 . . . ). For each of the dielectric-barrier discharge lamps (B, B1, B2 . . . ), with the power supply front stage (M) providing a common direct current power supply voltage (1) to each of the power supply back stages (S, S1, S2 . . . ) and each of the power supply back stages (S, S1, S2 . . . ) transforming the direct current power supply voltage (1) to an alternating current high voltage (H) of roughly periodic waveform by means of a switching element and a step-up transformer (T), and impressing that alternating current high voltage (H) on the electrodes (Ea, Eb) of the corresponding dielectric-barrier discharge lamps (B, B1, B2 . . . ), such that the frequency of the alternating current high voltage (H) of each of the power supply back stages (S, S1, S2 . . . ) is independently adjustable and the common direct current power supply voltage (1) of the power supply front stage (M) is adjustable.
(2) Dielectric-barrier discharge lamp light source equipment, in which the power supply front stage (M) supplies as common direct current power supply voltages both a direct current power supply voltage (2) for regular operation and a direct current power supply voltage (3) for lamp ignition to each of the power supply back stages (S, S1, S2 . . . ), with each of the power supply back stages (S, S1, S2 . . . ) having a lamp voltage selection switch circuit (X) to select either the direct current power supply voltage (2) for regular operation or the direct current power supply voltage (3) for lamp ignition, and the lamp voltage selection switch circuit (X) selecting the direct current power supply voltage (2) for regular operation when the dielectric-barrier discharge lamp is turned on or direct current power supply voltage (3) for lamp ignition during regular operation, such that the common voltage is transformed to an alternating current high voltage (H) of roughly periodic waveform based on the voltage selected.
(3) Dielectric-barrier discharge lamp light source equipment, in which the power supply front stage (M) supplies as common direct current power supply voltages both a modulated low-level direct current power supply voltage (4) and a modulated high-level direct current power supply voltage (5) to each of the power supply back stages (S, S1, S2 . . . ), with each of the power supply back stages (S, S1, S2 . . . ) having a lamp voltage selection switch circuit (Y) to select either the modulated low-level direct current power supply voltage (4) or the modulated high-level direct current power supply voltage (5), and the lamp voltage selection switch circuit (Y) cyclically alternating between two states, namely the state when the modulated low-level direct current power supply voltage (4) is selected and the state when the modulated high-level direct current power supply voltage (5) is selected, such that the common voltage is transformed to an alternating current high voltage (H) of roughly periodic waveform based on the voltage selected, and the ratio of length of continuation of the two states can be adjusted.